The present invention relates to semiconductor devices such as a semiconductor optical device and a semiconductor electron device which utilize a strained-layer superlattice.
With the recent progress of thin film growth technology, it is now possible to grow alternately layers of semiconductor materials of different lattice constants without causing crystal defects when the layers are sufficiently thin. This structure is commonly referred to as a strained-layer superlattice or strained quantum well. The structure of the semiconductor device with the strained-layer superlattice in its active layer substantially raises the degree of freedom of selecting semiconductor materials, thereby permitting the growth, on ordinary clad layers, of materials of band gap energies, optical characteristics, electrical characteristics and the like which are unobtainable with a lattice matching system.
Conventional semiconductor devices with the strained-layer superlattice have a structure wherein well layers and barrier layers stacked alternately with each other, and at least one layers of the well layers and the barrier layers are formed of a semiconductor of a lattice constant greater than, substantially equal to, or smaller than that of the clad layer.
In case of fabricating the strained-layer superlattice by the epitaxial growth of sufficiently thin layers of materials having different lattice constants, the material of a large lattice constant is subjected to a compressive stress in the direction of the plane of the strained-layer superlattice and the material of a small lattice constant is subjected to a tensile stress in the direction of the plane of the strained-layer superlattice. As a result, the lattice constant of the strained-layer superlattice in the direction of its plane assumes a certain value included in a range between the both lattice constants. Now, let this value be represented as the lattice constant, in the plane direction, a.sub.SL in an imaginary state in which the strained-layer superlattice is floating in the free space, i.e. floating in space off the clad layer. FIGS. 7A and 7B show variations in the lattice constant by the stacking of layers. FIG. 7A shows a semiconductor A of a lattice constant a.sub.A and a semiconductor B of a lattice constant a.sub.B. FIG. 7B shows stacked semiconductors A and B of a common lattice constant a.sub.SL of the imaginary state in the free space and a clad layer C of the lattice constant a.sub.CL.
As depicted in FIG. 7B, in the case of stacking semiconductors of different lattice constants in the free space, the interface of the layers is subjected to a strain, but since the stresses applied to the strain-layer superlattice in its entirety are in equilibrium, it is possible to grow layers of the both materials repeatedly without causing dislocations and other defects. In practice, however, since the strained-layer superlattice is grown, not in the free space, but on a clad layer which is far thicker than each layers of the superlattice, the lattice constant of the strained-layer superlattice in its plane direction is constrained to become the same as the lattice constant of the clad layer, not the lattice constant a.sub.SL in the free space. In consequence, the entire strained-layer superlattice is stressed by the clad layer. Hence, the conventional semiconductor device having the strained-layer superlattice is defective such that as the number of layers forming the strained-layer superlattice increases, the overall stress imposed thereon increases to reach a critical film thickness at last, causing defects. Thus, it is difficult to grow a strained-layer superlattice having many layers. Materials of widely different lattice constants cannot be used for semiconductors A and B, that is, the degree of freedom for selecting the materials for these two kinds of layers is low. Moreover, the use of particular pairs of materials, such as AlAs/InAs and GaAsP/InGaAs, may sometimes encounter difficulty in direct crystal growth.